1. Technical Field of the Invention
The present invention relates to a refractive element suitable for refracting x-ray beams of the type that comprises a material having sections removed. The invention also relates to a lens comprising the refractive elements.
2. Background of the Invention
WO 01/06518, by the same inventor and same applicant, relates to a refractive arrangement for X-rays, and specially to a lens comprising: a member of low-Z material. The low-Z material has a first end adapted to receive x-rays emitted from an x-ray source and a second end from which the x-rays received at the first end emerge. It further comprises a plurality of substantially triangular formed grooves disposed between the first and second ends. The plurality of grooves are oriented such that, the x-rays which are received at the first end, pass through the member of low-Z material and the plurality of grooves, and emerge from the second end, are refracted to a focal line.
The aperture of a Multi-Prism Lens (MPL) or a.k.a. saw-tooth refractive lens, e.g. as described in WO 01/06518, is limited by absorption of the beam in the lens material. The intensity transmission function of the lens is Gaussian with a root-mean-square (rms) width given byσabs=√{square root over (Fδι)},  (1),where F is the focal length, δ is the decrement of the real part of the index of refraction, and l is the attenuation length. The aperture in turn limits the possible intensity gain and diffraction-limited resolution. Apart from the focal length, the aperture is only a function of the material properties, and is thus a true physical limit. Choosing a material with lowest possible atomic number maximizes it. Until now, various polymers, diamond, beryllium, silicon and lithium have been used as lens materials. The choice of material is of course also restricted by available fabrication methods and is furthermore a cost issue.
The focusing power of a lens is a function of the phase-shift of the outgoing wave. If a cylindrical wave (=phase-shift) is created, the wave will converge to a line focus. In a regular MPL, for a large portion of the lens aperture, the wave is phase-shifted much more than 2π (or 360°). In other words, rays will pass a thickness of material larger than the 2π-shift length given byL2π=λ/δ  (2).
This length is of the order of 10-100 μm for hard x-rays; λ is the wavelength.